1. Field of the Invention
The present invention generally relates to the control and regulation of fluid pressure, and more specifically to a spring-loaded pressure regulating valve including a rolling diaphragm and compensation for variation of the spring force with diaphragm displacement.
2. Description of the Related Art
It is required in many applications to regulate the pressure of a fluid such as air in a chamber or conduit to a predetermined value. For example, a high pressure air reservoir may be provided in a wheel on which a tire for an automotive vehicle is mounted, and a pressure regulating valve disposed between the reservoir and the tire. When the tire pressure drops below a predetermined value, the valve opens and allows air from the reservoir to flow into the tire. When the tire pressure has risen to the predetermined value, the valve closes and stops the supply of air from the reservoir. A typical system of this type is disclosed in U.S. Pat. No. 4,967,376, entitled "TIRE PRESSURE WARNING SYSTEM", issued Jan. 10, 1978 to W. Barabino.
A conventional pressure regulating valve such as disclosed in the Barabino patent includes a control element in the form of a piston, diaphragm or bellows which has one end exposed to the chamber pressure which is to be regulated. A compression spring engages with the opposite end of the control element and urges it against the chamber pressure with a force corresponding to the predetermined pressure value. The control element actuates a valve which is connected between the high pressure reservoir and the chamber. As the chamber pressure drops below the predetermined value, the spring moves the valve element to an open position which establishes communication between the reservoir and the chamber, and vice-versa.
A conventional spring-loaded pressure regulating valve is disadvantageous in that the regulated pressure decreases as the fluid flow rate increases. More specifically, the valve must be opened to a greater extent to enable an increase in flow rate. This causes the compression spring to expand and exert a smaller force on the control element in accordance with Hooke's Law, F=KX, where F is the spring force, X is the spring compression (displacement of one end of the spring relative to the opposite end) and K is the "spring constant" which corresponds to the stiffness of the spring.
The force exerted on the control element by the fluid pressure in the chamber is equal to F=PA, where P is the chamber pressure and A is the cross-sectional area of the control element exposed to the pressure P in opposition to the spring force. Combining these equations produces P =KX/A. Since K and A are constant, the pressure P to which the air in the chamber is regulated is proportional to the spring compression X, and decreases as the valve opens and the spring expands. Thus, a conventional pressure regulating valve disadvantageously regulates the chamber pressure to a progressively lower value as the fluid flow rate increases and the valve opens to a larger extent.